Molecular analyses of Xenopus laevis Mesp-related genes.

During vertebrate somitogenesis, somites bud off from the anterior end of the presomitic mesoderm (PSM). Mesodermal posterior (Mesp)-related genes play essential roles in somitogenesis, particularly in the definition of the somite boundary position. Among vertebrates, two types of Mesp-related genes have been identified: Mesp1 and Mesp2 in the mouse; Meso-1 and Meso-2 in the chicken; Xl-mespa and Xl-mespb (also known as Thylacine1) in the African clawed frog (Xenopus laevis); and mesp-a and mesp-b in the zebrafish. However, the functional differences between two Mesp-related genes remain unknown. In the present study, we carried out comparative analyses of the Xl-mespa and Xl-mespb genes. The amino acid sequences of the Xl-mespa and Xl-mespb proteins showed a high level of similarity. The expression of Xl-mespa started broadly in the ventrolateral mesoderm and gradually shifted to a striped pattern of expression. In contrast, Xl-mespb showed a striped pattern of expression from the start. These expression profiles completely overlapped at the PSM during somitogenesis. To investigate the functional differences between Xl-mespa and Xl-mespb in terms of target gene regulation, we carried out a luciferase assay using the murine Lunatic fringe (L-fng) promoter. Transcription of the L-fng promoter was activated more strongly by Xl-mespb than by Xl-mespa. This same pattern was observed for the murine Mesp-related proteins. These results suggest that the functional differences between the two types of Mesp-related genes are evolutionally conserved in vertebrates.